Instance Pattern - Enforce Instance Fields Similar to Interfaces

[davemcdowell]

The 'Instance Pattern' would be a kind of marker or contract much like an interface, but for instance fields, not methods. A declaration-only contract for instance fields, in the way that interfaces work for methods. This would allow us to enforce that certain fields exist without forcing a class into a rigid inheritance structure.

Right now we can only use rigid inheritance or convoluted dependency injection to achieve something of this nature. Instance Patterns would allow a contract to be formed with a class that ensures the existence of specific instance fields. You could try to enforce this type of pattern with reflection, but that gets messy quickly.

-See below franchyd recommends using interfaces with auto-properties rather than all this trouble! Thanks Franchy!

public InstancePattern ExamplePattern
{
    int FieldA { get; set; }
    string FieldB { get; set; }
}

public class MyClass, ExamplePattern
{
    // Enforces that FieldA and FieldB exist in the class
    public int FieldA { get; set; }
    public string FieldB { get; set; }
}

[HaloFour]

What does this "instance pattern" do aside enforce that the marked class have said members? Can you use it as a type in it's own right? Can you use it as a generic type argument or a constraint? Can you have a variable of the pattern, and reference the fields through it?

[davemcdowell]

What does this "instance pattern" do aside enforce that the marked class have said members?
They simply enforce the existence of specific fields within a class. This provides a way to define a contract for fields without imposing a full inheritance structure like an interface does for methods.

Instance Patterns would provide:
-Easier type-checking: Devs can define instance patterns contractually with classes much how methods are handled with interfaces.
-Semantic Clarity: If a class implements an instance pattern that field pattern is guaranteed to exist without the need to force them into rigid inheritance structures.

Can you use it as a type in it's own right?
Yes, you could likely use them as types in their own right something like below, if an interface can do it an instance pattern can do it. This could allow you to reference objects of types that fulfill the instance pattern contract like below:

public class MyClass, ExamplePattern
{
   public int FieldA { get; set; }
   public string FieldB { get; set; }
}

ExamplePattern patternInstance = new MyClass();

Can you use it as a generic type argument or a constraint?
Yes, that would make the most sense and once again if an interface can do it then the instance pattern would need to mimic this behavior as well. In the example below T would be any class that implements the ExamplePattern (IP from above) contract and guarantee that the fields FieldA and FieldB are available.

public void PrintFields<T>(T obj) where T : ExamplePattern
{
    Console.WriteLine(obj.FieldA);
    Console.WriteLine(obj.FieldB);
}

Can you have a variable of the pattern, and reference the fields through it?
Yes, you'd be able to reference those fields from an object that implements the pattern, similar to how you reference methods through Interfaces.

public class MyClass, ExamplePattern
{
    public int FieldA { get; set; }
    public string FieldB { get; set; }
}

ExamplePattern instance = new MyClass();
Console.WriteLine(instance.FieldA);  // Access FieldA through the pattern

I hope this shed some more light on the proposal, I think this should have covered what you asked, let me know!

[franchyd]

Its not clear exactly how this is different than an interface.

[davemcdowell]

Interfaces cannot have instance fields.

Imagine you have several classes, each with their own specific base class or even none at all. But all these classes need to store certain fields (say FieldA and FieldB), and you don’t want to force them to inherit from a common base class just to meet this requirement.

With an Instance Pattern, you could create a pattern that enforces the existence of these fields without changing the class’s inheritance hierarchy.

Sorry if I'm mistaken or going in a bad direction, Interfaces only deal with defining property signatures and not the actual backing field / instance field correct?

[HaloFour]

Correct, fields are not considered part of the contract of a type. Per the .NET guidelines:

https://learn.microsoft.com/en-us/dotnet/standard/design-guidelines/field

❌ DO NOT provide instance fields that are public or protected.

It's better to expose them as properties, which provides more flexibility and encapsulation.

[davemcdowell]

Major Difference Between the Two:

1. With Interfaces:

   An interface only defines the behavior (i.e., properties with getters and setters).
   You still have to manually provide a backing field in the implementing class to store the data for the property.
   Interfaces don’t directly enforce that fields are present in the class; they just ensure that there are getters and setters.

2. With Instance Patterns:

   An Instance Pattern defines a contract for fields (data) directly.
   It enforces the presence of specific fields within the implementing class, ensuring that certain data members (fields) exist as part of the class's structure. You don’t need to manually create backing fields for properties; it ensures those fields are directly part of the class.

Am I going in a bad direction for a proposal? it's my first and maybe it's easier to do this with interfaces and backing fields?

I do see that the existing interface route is way more inline with encapsulation principles. I just feel like there could be something to this Instance Pattern, although if someone defined fields in an instance pattern to public then that would for sure break encapsulation. I guess you'd have to enforce that fields in an instance pattern must be private or protected and always defined with accessors.

Hmm food for thought, but I guess I'll stick with backing fields and interfaces for now; thanks for the chat though - maybe someone else will get an idea from all this.

[franchyd]

Major Difference Between the Two:

1. With Interfaces:
   An interface only defines the behavior (i.e., properties with getters and setters).
   You still have to manually provide a backing field in the implementing class to store the data for the property.
   Interfaces don’t directly enforce that fields are present in the class; they just ensure that there are getters and setters.
2. With Instance Patterns:
   An Instance Pattern defines a contract for fields (data) directly.
   It enforces the presence of specific fields within the implementing class, ensuring that certain data members (fields) exist as part of the class's structure. You don’t need to manually create backing fields for properties; it ensures those fields are directly part of the class.

Am I going in a bad direction for a proposal? it's my first and maybe it's easier to do this with interfaces and backing fields?

I do see that the existing interface route is way more inline with encapsulation principles. I just feel like there could be something to this Instance Pattern, although if someone defined fields in an instance pattern to public then that would for sure break encapsulation. I guess you'd have to enforce that fields in an instance pattern must be private or protected and always defined with accessors.

Hmm food for thought, but I guess I'll stick with backing fields and interfaces for now; thanks for the chat though - maybe someone else will get an idea from all this.

This is valid CSharp code today. Do you want to enforce that an implementer can only use auto properties and not be implemented with manual properties? Why does that matter?

public interface ExamplePattern
{
    int FieldA { get; set; }
    string FieldB { get; set; }
}

public class MyClass : ExamplePattern
{
    public int FieldA { get; set; } // Auto-property has a backing field implicitly
    public string FieldB { get; set; } // Auto-property has a backing field implicitly
}

[davemcdowell]

Thanks for the clarification! I think I was aiming for something a bit more explicit than what auto-property implicit backing fields provide.

I was trying to create a way that would allow users to define both the fields and accessors in one package - then easily apply that pattern to any class (like a retro-fitted contract for not only the signatures, but also backing fields with less boilerplate).

It looks like (auto-property implicit backing fields) address that completely, just in a more implicit way 😉 I appreciate the feedback!

*Edited for clarity and better wording

[jaredpar]

Correct, fields are not considered part of the contract of a type ...

For struct instance fields, even private ones, can very much be part of the type contract.

https://blog.paranoidcoding.org/2016/02/15/are-private-members-api-surface.html